Tallgrass prairies provide a valuable source of diverse native plant germplasm. Seed harvested from native prairies can be used to revegetate highly erodible or marginal cropland and degraded rangeland if adequate quantities of seed can be produced. The effect of spring burning, fertilization, and atrazine [6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-diamine] on big bluestem (Andropogon gerardii Vitman var. gerardii Vitman) and indiangrass [Sorghastrum nutans (L.) Nash] reproductive stem density and seed production was determined in 9 tallgrass prairie environments (each year by site was considered a unique environment). Studies were conducted at tallgrass prairies near Bloomfield, Lincoln, and Virginia Nebr., from 1987 through 1990. Atrazine was applied at rates of 0 and 2.2 kg a.i. ha-1 in mid-spring. Fertilizer rates applied in late spring were 0 and 110-0 kg N-P ha-1 at Lincoln in 1987, 0 and 110-22 kg N-P ha-1 at Bloomfield in 1987 and all sites in 1988, and 0 and 67-22 N-P kg ha-1 at all sites in 1989 and 1990. Improvement practices increased stem density of big bluestem in 5 environments and indiangrass in 4 environments. Number of germinable seed produced by the grasses was influenced by treatment only in 1987 and 1990 when precipitation amounts were above or near the long-term average. In 1987, atrazine increased indiangrass germinable seed number from 202 to 481 seed m-2 at Bloomfield. At Lincoln in 1987, the combined effects of fire, fertilizer, and atrazine increased the number of germinable indiangrass seed to 2,517 seed m-2 as compared to 331 seed m-2 produced on areas that had not been treated. In 1990, burning in mid-May increased big bluestem seed number from 52 to 125 germinable seed m-2 at Virginia, and fertilizer increased big bluestem seed number from 333 to 724 seed m-2 at Lincoln. The tallgrasses did not produce germinable seed in 1988 and 1989, presumably because of drought conditions that persisted both years